Someone made an excellent version of Yukari's fashion show dress, try searching the galleries. I know we talked about it here: http://livejournal.com/~parakiss
As for whether you could finish it in time, that depends on your skills and how much time you have to devote to it. If you're determined you can, it's still February. If you don't have much sewing experience it may be easier to buy a dress for the base then add things like the roses, accessories, etc.

Try searching the forums and gallery for 'prom', I know several people here have cosplayed for it. Lots of series have formal outfits, so it's really a matter of choosing your favorite design. Good luck!